This invention relates to devices and methods for coating substrates and for improving the uniformity of non-uniform or defective coatings.
There are many known methods and devices for coating a moving web and other fixed or moving substrates. Several are described in Booth, G. L., xe2x80x9cThe Coating Machinexe2x80x9d, Pulp and Paper Manufacture, Vol. 8, Coating, Converting and Processes, pp 76-87 (Third Edition, 1990). For example, gravure roll coaters (see, e.g. U.S. Pat. No. 5,620,514) can provide relatively thin coatings at relatively high run rates. Attainment of a desired specific average caliper usually requires several trials with gravure rolls of different patterns. Runtime factors such as variations in doctor blade pressure, coating speed, temperature, or liquid viscosity can cause overall coating weight variation and uneven localized caliper in the machine or transverse directions.
Barmarks and chatter marks are bands of light on heavy coating extending across the web. These are regarded as defects, and can be caused by factors such as vibration, flow pulsation, web speed oscillation, gap variation and roll drive oscillation. Chatter marks are commonly periodic, but barmarks can occur as the result of random system upsets. Gutoff and Cohen, Coating and Drying Defects (John Wiley and Sons, New York, 1995) discusses many of the sources of cross web marks and emphasizes their removal by identifying and eliminating the fundamental cause. This approach can require substantial time and effort.
Multiple lane coaters include those shown in U.S. Pat. Nos. 3,920,862; 5,599,602; 5,733,608 and 5,871,585. Gravure coating can also be used to produce down web lanes of a single formulation at a coating station, by using spaced circumferential patterns on the gravure roll or circumferential undercuts on the web back up roll. However, due to intermixing that occurs at the nip, abutting lanes of different formulations can not be applied from the same gravure roll.
Under some gravure roll coating run conditions, a gravure roll pattern appears in the wet coating. Gravure roll marks can be removed with an arcuate flexible smoothing film located down web from the gravure roll (see, e.g., U.S. Pat. No. 5,447,747); with a smoothing roll or rolls bearing against an intermediate coating roll (see, e.g., U.S. Pat. No. 4,378,390) or with a set of smoothing rolls located down web from the gravure roll (see, e.g., U.S. Pat. No. 4,267,215). In Examples 1-7 and 10 of the ""215 patent, a continuous coating was applied to a plastic film and subsequently contacted by an undriven corotating stabilizing roll 68 and a set of three equal diameter counter rotating spreading rolls 70. The respective diameters of the stabilizing roll and spreading rolls are not disclosed but appear from the Drawing to stand in a 2:1 ratio. In Example 10 of the ""215 patent, the applicator roll speed was increased until the uniformity of the coating applied to the web began to deteriorate (at a peripheral applicator roll speed of 0.51 m/s) and surplus coating liquid began to accumulate on the web surface upstream of the rolls 70 (at a peripheral applicator roll speed of 0.61 m/s). Coatings having thicknesses down to 1.84 micrometers were reported.
Several coaters having brush or roller smoothing devices are also shown in the above-mentioned Booth article.
Very thin coatings (e.g., about 0.1 to about 5 micrometers) can be obtained on gravure roll coaters by diluting the coating formulation with a solvent. Solvents are objectionable for health, safety, environmental and cost reasons.
Multiroll coaters (see, e.g., U.S. Pat. Nos. 2,105,488; 2,105,981; 3,018,757; 4,569,864 and 5,536,314) can also be used to provide thin coatings. Multiroll coaters are shown by Booth and are reviewed in Benjamin, D. F., T. J. Anderson, and L. E. Scriven, xe2x80x9cMultiple Roll Systems: Steady-State Operationxe2x80x9d, AIChE J., V41, p. 1045 (1995); and Benjamin, D. F., T. J. Anderson, and L. E. Scriven, xe2x80x9cMultiple Roll Systems: Residence Times and Dynamic Responsexe2x80x9d, AIChE J., V41, p. 2198 (1995). Commercially available forward-roll transfer centers typically use a series of three to seven counter rotating rolls to transfer a coating liquid from a reservoir to a web via the rolls. These coaters can apply silicone release liner coatings at wet coating thickness as thin as about 0.5 to about 2 micrometers. The desired coating caliper and quality are obtained by artfully setting roll gaps, roll speed ratios and nipping pressures.
U.S. Pat. No. 4,569,864 describes a coating device in which a thick, continuous premetered coating is applied by an extrusion nozzle to a first rotating roll and then transferred by one or more additional rolls to a faster moving web. The extrusion nozzle is placed very close to the first roll (e.g., 25 to 50 micrometers) in order to obtain an even and smoothly distributed coating on the first roll.
U.S. Pat. No. 5,460,120 describes a coating device in which a coating is spray-applied to the underside of a moving web immediately upstream from a resilient, compressible, saturable applicator.
Electrostatic spray coating devices (see, e.g., U.S. Pat. Nos. 4,748,043; 4,830,872; 5,326,598; 5,702,527 and 5,954,907) atomize a liquid and deposit the atomized droplets assisted by electrostatic forces. In some applications the desired coating thickness is larger than the droplet diameter and the droplets just land on top of each other and coalesce to form the coating. In other applications the desired coating thickness is smaller than the droplet diameter. For these thin film coatings a solvent can be used, but if a solventless coating is desired, then the drops must land on the web some distance apart from each other in order to satisfy the small volume requirement of the thin film coating. Then the droplets must spread in order to merge into a continuous voidless coating. Spreading takes time and can be a rate-limiting step for these electrostatic spray coating processes. If the surface chemistry is such that the liquid does not sufficiently spread on the substrate in the available time before cure or hardening, then voids will remain in the coating.
The present invention provides, in one aspect, a method for improving the uniformity of a wet coating on a substrate comprising contacting the coating at a first position with wetted surface portions of:
a) three or more periodic pick-and-place devices, or
b) two or more rotating periodic pick-and-place devices having the same direction of rotation and re-contacting the coating with such wetted surface portions at positions on the substrate that are different from the first position and not periodically related to one another with respect to their distance from the first position. The placement positions of the pick-and-place devices are not periodically related (that is, they are not the same or integer multiples of one another) so that their actions do not reinforce coating defects along the substrate.
The invention also provides a method for applying a coating to a substrate comprising applying to the substrate an uneven wet coating, contacting the coating at a first position with wetted surface portions of:
a) three or more periodic pick-and-place devices, or
b) two or more rotating periodic pick-and-place devices having the same direction of rotation
and re-contacting the coating with such wetted surface portions at positions on the substrate that are different from the first position and not periodically related to one another with respect to their distance from the first position.
In another aspect, the invention provides a method for coating at least one lane comprising at least one coating on a substrate, and for optionally abutting more than one of such lanes without substantial intermixing of the coatings in the lanes.
The invention also provides devices for carrying out such methods. In one aspect, the devices of the invention comprise an improvement station comprising two or more pick-and-place devices that can periodically contact and re-contact a wet coating at different positions on a substrate, wherein the periods of the devices are selected so that the uniformity of the coating is improved. In a preferred embodiment, the improvement station comprises three or more rolls having different rotational periods. In another aspect, the devices comprise a coating apparatus for applying an uneven (and preferably discontinuous) coating to a substrate and an improvement station comprising two or more of the above-mentioned pick-and-place devices for contacting and re-contacting the coating at different positions on the substrate whereby the coating becomes more uniform on the substrate. In yet a further aspect, the invention provides an apparatus comprising a coating station for applying an uneven (and preferably discontinuous) coating to a first substrate, an improvement station comprising two or more of the above-mentioned pick-and-place devices for contacting and re-contacting the coating at different positions on the first substrate whereby the coating becomes more uniform on such first substrate, and a transfer station for transferring the uniform coating from the first substrate to a second substrate. In a further aspect, this latter apparatus comprises a coating station that coats at least one lane on said first substrate and a transfer station that transfers such lane to said second substrate.
The methods and devices of the invention also facilitate much more rapid drying of wet coatings on a substrate. Thus in a further aspect, the methods of the invention further comprise drying the coating, and the devices of the invention include a drying station having a plurality of pick-and-place devices that contact and re-contact a substrate having an uneven wet coating, whereby the pick-and place devices increase the drying rate of the coating.
The methods of the invention can provide extremely uniform coatings and extremely thin coatings, at very high rates of speed. The devices of the invention are simple to construct, set up and operate, and can easily be adjusted to alter the coating thickness.